Re: eval-print-last-sexp: "Selecting deleted buffer" error when the current buffer is killed evaluating the expression

[Eli Zaretskii]

> Date: Fri, 30 Jul 2021 16:07:53 +0200
> From: Matthew White <mehw.is.me@inventati.org>
> 
> > Could you please tell what are the real-life situations where this
> > problem happens and causes trouble?  Killing the current buffer this
> > way is unusual, so why isn't what we have now sufficient?  You tried
> > to insert something into a killed buffer, Emacs told you it cannot do
> > that; why is that a problem?
> 
> I understand that such occasions are rare, I mean killing the current
> buffer evaluating an expression also in the current buffer like:
> 
> (kill-buffer (current-buffer))
> 
> eval-print-last-sexp is trying to insert something into a dead buffer,
> by calling terpri with standard-output set to the dead buffer.
> 
> This could be prevented by checking standard-output:
> 
> (when (buffer-live-p standard-output)
>   (terpri))
> 
> Less uncommon than killing the current buffer evaluating an expression
> with C-j, is redirecting the output to the echo area with a '-' prefix
> C-u - C-j.  I'd expect no output into the current buffer, and the call
> to eval-last-sexp respects this, while terpri does not and prints into
> the standard-output (aka current buffer at the time C-j was pressed).
> 
> There's also the strange behavior of printing to a buffer which is not
> the one where the expression was evaluated. This is also rare, happens
> when the buffer is killed, like described previously:
> 
> ;; Always prints to the actual current buffer.
> (eval-last-sexp (or eval-last-sexp-arg-internal t))
> 
> > 
> > IME, trying to "fix" such obscure problems causes problems of its own
> > that we then get to rectify for several releases in the future, so the
> > net gains are very small if there are any.
> 
> I agree, there is a little gain into that.  I just explored the source
> of the problems to understand what they are.
> 
> IMHO, when C-u - C-j is used terpri shouldn't print to stadard-output,
> since the evaluation's return value is destined to the echo area.  And
> checking if standard-output is a live buffer is also trivial.
> 
> What do you think?

I think we should leave this issue alone, but maybe someone else has a
different opinion and a rationale to go with that?